PURPOSE: To obtain a digital filter with a low cost by supplying alternately a new data to two storage areas performing the function of the delay element group of the digital filter and updating the oldest data in each storage area.

CONSTITUTION: A latch circuit 51 keeps supplying an input data for a write period to RAMs 51b, 52b. A controller 47a is provided with the address designation patterns of a number in response to the address capacity of RAMs A, B and its generating means. A new data held in the latch 51a is alternately fetched in the RAMs A, B in the data write routine and the new data is stored in the address of the oldest data stored in the RAMs A, B. The addition term between odd numbered data and even numbered data is used as the unit of addition in the integration value calculation, they are separated in advance into the odd and even number data and stored separately in the two RAMs. When corresponding data pair is read, the data transfer between the RAMs is not required.